Delay demodulation devices

ABSTRACT

Delay demodulation devices with optimized temperature and pressure distribution on a Planar Lightwave Circuit (PLC) are provided. The delay demodulation device  1  comprises: an input waveguide  2,  which receives DQPSK signals; a Y-shape waveguide  3 , which splits the light waveguide  2;  a first Mach-Zehnder interferometer  4;  and a second Mach-Zehnder interferometer  5.  Both ends of arm waveguides  8, 9  on Mach-Zehnder interferometer  4  and the both ends of arm waveguides  12, 13  on Mach-Zehnder interferometer  5  are bent toward the center portion of the PLC. Because of the angle, the lengths of these arm waveguides  8, 9  are shortened in the Z-direction, and input couplers  6, 10  and output couplers  7, 11  of the Mach-Zehnder interferometers are shortened as well in the Z-direction. Therefore, the area covered by the Mach-Zehnder interferometers  4, 5  is reduced.

TECHNICAL FIELD

The present invention relates to delay demodulation devices used for optical fiber communication, and particularly, relates to delay demodulation devices equipped with Planar Lightwave Circuit (PLC) type Mach-Zehnder interferometers, which demodulate Differential Quadrature Phase Shift Keying (DQPSK) signals in Dense Wavelength Division Multiplexing (DWDM) transmission.

RELATED ARTS

Recently, with the rapid growth in broadband networks, high speed optical transmission systems (toward transmission rate of 40 Gbps) are being investigated actively. However, when the transmission rate is increased, the time duration per 1 bit of optical signal is reduced and, because of the characteristics of an optical fiber, wave shape is deteriorated, and therefore the quality of a communication line is deteriorated. For 40 Gbps-class long distance transmission, transponders that transform an optical signal to an electrical signal and then transform the electrical signal back to an optical signal are needed in the transmission path. Therefore, it is difficult to create a high speed optical transmission system using existing optical fiber networks.

Because of this issue, research and development has been done in Differential Quadrature Phase Shift Keying (DQPSK). DQPSK reduces deterioration of signal-wave profile by increasing the time duration per bit of an optical signal.

DQPSK transmits four symbols of information as four corresponding optical phase shifts. In other words, each symbol of information corresponds to a value (0, 1, 2 or 3), which comprises two bits of data, and the symbols of information are transmitted by shifting the phase of carrier wave between adjacent symbols by an amount (θ, θ+π/2, θ+π or 0+3π/2) determined by the pair of bits to be transmitted. 40 Gbps DQPSK transmission can transmit four times longer distance than conventional 40 Gbps transmission. Because of DQPSK, it is believed that construction of networks between large cities can be achieved using existing optical fiber networks.

For example, conventional delay demodulation devices, which demodulate in receiving devices by using DQPSK signals or Differential Phase Shift Keying (DPSK) signals, are disclosed in Japanese Patent Application Laid-open 2007-60442, and in Japanese Patent Application Laid-open 2007-151026.

Photo receiving circuits disclosed in Japanese Patent Application Laid-open 2007-60442 are equipped with Mach-Zehnder interferometers, which propagate return-to-zero (RZ) modulated DPSK signals through a pair of optical paths, which are equipped with a one-symbol delay element in one of the pair optical paths.

Also, demodulation devices disclosed in Japanese Patent Application Laid-open 2007-151026 use Michelson interferometers to demodulate DPSK or DQPSK optical signals.

When delay detection is performed in 40 Gbps DQPSK signals, two PLC-type Mach-Zehnder interferometers are used to delay one symbol period (period for two bits). At that time, because free spectral range (FSR) is about 20 GHz, difference in length between the two Mach-Zehnder interferometers increases, and therefore, circuit layout becomes relatively large. If the circuit layout is large, the temperature distribution along the PLC surface becomes uneven, and the center wavelengths can be shifted easily due to environment and temperature fluctuations. Also, when stress distribution within the PLC surface becomes large, initial polarization dependent wavelength (PDλ) increases.

BRIEF SUMMARY OF THE INVENTION

The present invention seeks to overcome the above-identified problems. The purpose of the present invention is to provide delay demodulation devices with optimized temperature and pressure distribution on a Planar Lightwave Circuit (PLC).

To solve the above issue, a Planar Lightwave Circuit (PLC)-type delay demodulation device for demodulating Differential Quadrature Phase Shift Keying (DQPSK) signal is invented. The delay demodulation device comprises an input waveguide, which receives the DQPSK signals; a Y-shape waveguide, which splits the input waveguide; and first and second Mach-Zehnder interferometers. The first Mach-Zehnder interferometer comprises an input coupler, which is connected to one of the two waveguides split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths with respect to each other and connected between the input coupler and the output coupler. The second Mach-Zehnder interferometer comprises an input coupler, which is connected to the other waveguides split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths with respect to each other and connected between the input coupler and the output coupler. Both ends of the arm waveguides on the first Mach-Zehnder interferometer and both ends of the arm waveguides on the second Mach-Zehnder interferometer are bent toward the center portion of the PLC.

According to the construction above, if the direction of DQPSK signal propagates in the input waveguide is considered to be Z-direction, lengths of two arm waveguides on each Mach-Zehnder interferometer in the Z-direction are shortened. Accordingly, the directions of the input couplers and the output couplers of the Mach-Zehnder interferometers (i.e. the direction of DQPSK signals transmitted in the couplers) are tilted with respect to the Z-direction; therefore, lengths of the input couplers and the output couplers on the Mach-Zehnder interferometers in the Z-direction are shortened.

Because lengths of the two arm waveguides on each Mach-Zehnder interferometer in the Z-direction and lengths of the input couplers and the output couplers on Mach-Zehnder interferometers in the Z-direction are shortened, areas covered by the first and second Mach-Zehnder interferometers can be made smaller; and therefore, the PLC delay demodulation device can be made smaller. This also reduces temperature and stress distributions within the PLC. Therefore, the delay demodulation devices with little or no wavelength shift due to the environment and temperature fluctuation and small initial PDλ can be made.

In DQPSK signals, each symbol of information corresponds to a value (0, 1, 2 or 3), which comprises two bits of data, and the symbols of information are transmitted by shifting the phase of carrier wave between adjacent symbols by an amount (θ, θ+π/2, θ+π or θ+3π/2) determined by the pair of bits to be transmitted. Also, the PLC is a circuit, which includes an input waveguide, a Y-shape waveguide, and first and second Mach-Zehnder interferometers.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, one of the two waveguides split by the Y-shape waveguide is bent upward and the other waveguide is bent downward to create a space between the two; wherein the waveguide bent upward is connected to an input end of the input coupler of the first Mach-Zehnder interferometer, and the waveguide bent downward is connected to an input end of the input coupler of the second Mach-Zehnder interferometer; and the first and second output waveguides connected to the output coupler of the first Mach-Zehnder interferometer are bent downward, and the third and fourth output waveguides connected to the output coupler of the second Mach-Zehnder interferometer are bent upward to come close to each other.

According to the construction above, because the length of the two waveguides split by the Y-shape waveguide and the length of the four output waveguides on the Z-direction are shortened, the PLC of the delay demodulation device can be made smaller to further reduce temperature and stress distributions within the PLC.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, one of the two arm waveguides of each Mach-Zehnder interferometer has a difference in length Δ L with respect to the other arm waveguide of each Mach-Zehnder interferometer to create a phase shift of the DQPSK signal in one arm waveguide delay of π radians with respect to the other.

According to the construction above, after the adjustment, the delay demodulation device I performs phase shift control to shift the phases of the first Mach-Zehnder interferometer by π/2 radians to the phases of the second Mach-Zehnder interferometer, therefore, the optical signals (optical intensity signals) from the four output waveguides (four output ports) are shifted by π/2 radians with respect to the adjacent optical signals.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the input waveguide, the Y-shape waveguide, and the first and second Mach-Zehnder interferometers are waveguides placed on a base plate.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the base plate has an approximately square planar shape.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the first Mach-Zehnder interferometer is at the upper center portion of the PLC, and the second Mach-Zehnder interferometer 5 is at the lower center portion of the PLC.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, the center portion of the arm waveguides of the first Mach-Zehnder interferometer and the center portion of the arm waveguides of the second Mach-Zehnder interferometer are extended in parallel to each other.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, a first delay portion is placed in the first and second output waveguides to match the lengths of those two output waveguides, and a second delay portion is placed in the third and fourth output waveguides to match the lengths of those two output waveguides.

According to the construction above, the first delayed portion matches the length of the first output waveguide with the second output waveguide, and the second delayed portion matches the length of the third output waveguide with the fourth output waveguide. Therefore, the optical signals from the four output waveguides are shifted by π/2 radians with respect to the adjacent optical signals. Because of that, four-fiber fiber array can be directly connected to one side of the PLC chip, which includes the outputs of the waveguides.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, at least one heater is placed on at least one of the two arm waveguides of each Mach-Zehnder interferometer.

According to the construction above, by using the heaters at least one of the first and the second Mach-Zehnder interferometer or both of them, after the adjustment, the delay demodulation device can be performed phase shift control to shift the phases of the first Mach-Zehnder interferometer by π/2 radians to the phases of the second Mach-Zehnder interferometer.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, a half-wave plate is inserted at the center portion of each Mach-Zehnder interferometer.

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, a groove for inserting the half-wave plate, which passes through the center portions of the first and second Mach-Zehnder interferometers, is placed on the opposite side of the input waveguide with respect to the Y-shape waveguide.

According to the construction above, leaked light from the Y-shape waveguide 3 can be blocked. This prevents recombination of leaked light (DQPSK signal) from the Y-shape waveguide 3 with outputs of the output waveguides (output ends of the waveguides 43˜46).

According to another a Planar Lightwave Circuit (PLC)-type delay demodulation device, a portion facing to the Y-shape waveguide is filled with a resin.

According to the construction above, the portion of the groove is filled with a resin to block the leaked light from the Y-shape waveguide more effectively.

The present invention can be provided delay demodulation devices with optimized temperature and pressure distribution on a Planar Lightwave Circuit (PLC).

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken into connection with the accompanying drawing wherein one example is illustrated by way of example, in which;

FIG. 1 is a plan view of a delay demodulation device in one embodiment of the present invention.

FIG. 2 is a block diagram of an optical transmission system with Differential Quadrature Phase Shift Keying (DQPSK).

FIG. 3 is a cross-sectional drawing taken along line X-X in FIG. 1.

FIG. 4 is a cross-sectional drawing taken along line Y-Y in FIG. 1.

FIG. 5 is a closeup view of a first delay portion of the delay demodulation device shown in FIG. 1.

FIG. 6 is a closeup view of Z portion shown in FIG. 1.

FIG. 7 is a result of temperature characteristics of the delay demodulation device shown in FIG. 1.

FIG. 8 is a graph showing a spectrum of the delay demodulation device disclosed in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Detailed description as follows;

Construction of the delay demodulation device is shown in FIG. 1 through FIG. 8.

FIG. 1 is a plan view of a delay demodulation device in one embodiment of the present invention, FIG. 2 is a block diagram of an optical transmission system with Differential Quadrature Phase Shift Keying (DQPSK). FIG. 3 is a cross-sectional drawing taken along line X-X in FIG. 1, FIG. 4 is a cross-sectional drawing taken along line Y-Y in FIG. 1. FIG. 5 is a closeup view of a first delay portion of the delay demodulation device shown in FIG. 1, FIG. 6 is a closeup view of Z portion shown in FIG. 1. FIG. 7 is a result of temperature characteristics of the delay demodulation device shown in FIG. 1, FIG. 8 is a graph showing a spectrum of the delay demodulation device disclosed in FIG. 1.

Delay demodulation device 1 shown in FIG. 1 is a Planar Lightwave Circuit (PLC)-type delay demodulation device, which demodulates Differential Quadrature Phase Shift Keying (DQPSK) signals. Delay demodulation device 1 is, for example, a 40 Gbps DQPSK delay demodulation device used in a 40 Gbps DQPSK optical transmission system shown in FIG. 2.

In the optical transmission system, DQPSK signals are transmitted from an optical transmitter 40 to an optical fiber transmission line 54. In DQPSK signals, each symbol of information corresponds to a value (0, 1, 2 or 3), which comprises two bits of data, and the symbols of information are transmitted by shifting the phase of carrier wave between adjacent symbols by an amount (θ, θ+π/2, θ+π or θ+3π/2) determined by the pair of bits to be transmitted. DQPSK signals transmitted from the optical fiber transmission line 54 to an optical receiver 50 are converted to optical signals with four different intensities by the delay demodulation device 1, and furthermore, the optical signals are converted to electric signals by balanced receivers 51 and 52. In a receiving electric circuit 53, various processes such as decryption process are performed.

Delay demodulation device 1 shown in FIG. 1 comprises an input waveguide 2, which receives DQPSK signals; a Y-shape waveguide 3, which splits the input waveguide 2; a first Mach-Zehnder interferometer 4; and a second Mach-Zehnder interferometer 5.

The first Mach-Zehnder interferometer 4 comprises: an input coupler 6 connected to one of the two waveguides, which are split by the Y-shape waveguide 3; an output coupler 7 connected to output ends of output waveguides; and two arm waveguides 8, 9, which are connected between the both couplers 6, 7. The two arm waveguides 8, 9 are different in lengths. Similarly, the second Mach-Zehnder interferometer 5 comprises: an input coupler 10 connected to the other waveguide, which is split by the Y-shape waveguide 3; an output coupler 11 connected to output ends of output waveguides; and two arm waveguides 12, 13, which are connected between the both couplers 10, 11. The two arm waveguides 12, 13 are different in lengths.

The input couplers 6, 10 and the output couplers 7, 11 are 2 inputs×2 outputs-type, 3 dB couplers (50% directional couplers). One end of the input coupler 6 of the first Mach-Zehnder interferometer 4 is connected to the waveguide 14, and one end of the input coupler 10 of the second Mach-Zehnder interferometer 5 is connected to the waveguide 15.

Also, the two output ends (a through port and a cross port) of the output coupler 7 of the first Mach-Zehnder interferometer 4 are connected to the first and second output waveguides 21, 22, respectively. In a similar fashion, the two output ends (a through port and a cross port) of the output coupler 11 of the second Mach-Zehnder interferometer 5 are connected to the third and fourth output waveguides 23, 24, respectively.

Also, in the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4, there is a difference in the length of the waveguides Δ L to make phase shift of the DQPSK signal in one end (i.e. the arm waveguide 8) delay by π radians with respect to the other (i.e. the arm waveguide 9). In a similar fashion, in the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5, there is a difference in the length of the waveguides Δ L to make the phase shift of the DQPSK signal in one end (i.e. arm waveguide 12) delay by 7C radians with respect to the other (i.e. arm waveguide 13).

Each ends of the arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4 and that of the arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 is bent toward the center portion of the PLC 1A (i.e. the center portion of the PLC chip 1B, which includes the PLC 1A) as shown in FIG. 1. Here, PLC is a circuit includes an input waveguide, a Y-shape waveguide, and first and second Mach-Zehnder interferometers, all made from fused silica. The delay demodulation device 1 comprising the PLC 1A is manufactured as follow.

With flame hydrolysis deposition (FHD), silica material (SiO₂-type glass particles), which makes a lower cladding layer and a core layer, is deposited on a PLC base plate 30 (such as a silica base plate) as shown in FIG. 3. Then, a glass coating made by the deposition is fused (and becomes transparent) by adding heat. Later, desired waveguides are created by photo lithography and reactive ion etching, and a upper cladding is created with FHD method. In FIG. 3, on the PLC base plate 30, a cladding layer is created by the lower cladding layer and the upper cladding layer, and the arm waveguides 8, 9 are created as the core layer inside of the cladding layer 31. The PLC base plate 30 is approximately in square planar shape as shown in FIG. 1.

Also, the two waveguides 14, 15 split by the Y-shape waveguide 3 are bent upward and downward, respectively, to create a space between the two as shown in FIG. 1. One of the split waveguides (i.e. the waveguide 14) is connected to one of the two input ends of the input cupler 6 of the first Mach-Zehnder interferometer 4, and the other split waveguide (i.e. the waveguide 15) is connected to one of the two input ends of the input cupler 10 of the second Mach-Zehnder interferometer 5. Here, if the direction, which DQPSK signal propagates in the input waveguide 2 is considered to be the Z-direction (as shown in FIG. 1), the waveguide 14 is approximately perpendicular to the Z-direction and bent upward with a curvature. On the other hand, the waveguide 15 is approximately perpendicular to the Z-direction and bent downward with a curvature.

Also, the two output ends (a through port and a cross port) of the output coupler 7 of the first Mach-Zehnder interferometer 4 are connected to the first and second output waveguides 21, 22, respectively. In a similar fashion, the two output ends (a through port and a cross port) of the output coupler 11 of the second Mach-Zehnder interferometer 5 are connected to the third and fourth output waveguides 23, 24, respectively. The first and second output waveguides 21, 22 and the third and fourth output waveguides 23, 24 are bent downward and upward, respectively, to come close to each other as shown in FIG. 1.

More particularly, the first and second output waveguides 21, 22 are extended downward with a curvature from the two output ends of the output coupler 7 and are approximately perpendicular to the Z-direction. Because of the curvature, length of the second output waveguide 22 is longer than that of the first output waveguide 21. In similar fashon, the third and fourth output waveguides 23, 24 are extended upward with a curvature from the two output ends of the output coupler 11 and are approximately perpendicular to the Z-direction. Because of the curvature, length of the third output waveguide 23 is longer than that of the fourth output waveguide 24.

The first Mach-Zehnder interferometer 4 is at upper center portion of the PLC 1A, and the second Mach-Zehnder interferometer 5 is at lower center portion of the PLC IA as shown in FIG. 1. More particularly, the first and second Mach-Zehnder interferometers 4, 5 are symetric to each other with respect to a virtual center line on the PLC chip 1B, for example a straight line extended from the input waveguide 2 extends toward the Z-direction.

The center portion of the arm waveguides 8, 9 of the Mach-Zehnder interferometer 4 and the center portion of the arm waveguides 12, 13 of the Mach-Zehnder interferometer 5 are extended in parallel to each other.

A first delay portion 41 is placed in the first and second output waveguides 21, 22 to match the lengths of those two output waveguides 21, 22 as shown in FIG. 1. The first delay portion 41 includes a waveguide 43 connected to the first output waveguide 21, and a waveguide 44 connected to the second output waveguide 22, which is longer than the first output waveguide 21 as shown in FIGS. 1 and 5. The two waveguides 43, 44 of the first delay portion 21 are curved upward convexity with respect to the above mentioned virtual center line. The waveguide 43 has two straight portions 43 a, 43 b to extend the waveguide compare to the waveguide 44 as shown in FIG. 5. The output ends of the output waveguides 43, 44 of the first delayed portion 41 are output ports (a first output port and a second output port), which output signals 1, 2 ((a) and (b) in FIG. 8), respectively, wherein the phase of one output signal is shifted by π radians with respect to the other.

Also, a second delay portion 42 is placed in the third and fourth output waveguides 23, 24 to match the lengths of those two output waveguides 23, 24. The second delay portion 42 includes a waveguide 45 connected to the fourth output waveguide 24, and a waveguide 46 connected to the third output waveguide 23, which is longer than the fourth output waveguide 24 as shown in FIG. 1. The two waveguides 45, 46 of the second delay portion 42 is curved downward convexity with respect to the above mentioned virtual center line. The waveguide 45 has two straight portions similar to the straight portions 43 a, 43 b in FIG. 5 to extend the waveguide compare to the waveguide 46. The output ends of output waveguides 46, 45 of the second delayed portion 42 are output ports (a first output port and a second output port), which output signals 3, 4 ((c) and (d) in FIG. 8), respectively, wherein the phase of one output signal is shifted by π radians with respect to the other.

The first Mach-Zehnder interferometer 4 includes a first heater A and a second heater C placed on the arm waveguide 8, and a third heater B and a fourth heater D placed on the arm waveguide 9. In similar fashion, the second Mach-Zehnder interferometer 4 includes a first heater E and a second heater G placed on the arm waveguide 12, and a third heater F and a fourth heater H placed on the arm waveguide 13. Heaters A˜H are placed above the corresponding arm waveguide, and the heaters are Tantalium compound thin layer heaters made by a weld slag, which are placed onto the upper cladding (the cladding layer 31 in FIG. 3). In FIG. 3, the heaters A, B placed above the cladding layer 31 of the arm waveguides 8, 9 are shown.

Also, to reduce PDλ, half-wave plates 47, 48 are inserted at the center portions of the Mach-Zehnder interferometers 4, 5, respectively, as shown in FIG. 1. Groove 49 for inserting the half-wave plates 47, 48 is placed not only on the center portions of the Mach-Zehnder interferometers 4, 5, but also on the straight line M-L in FIG. 1, which passes through the center portions of Mach-Zehnder interferometers 4, 5. Also, to blocks leaked light from the Y-shape waveguide 3, the groove 49 is placed on opposite side of the input waveguide 2 with respect to the Y-shape waveguide 3.

Furthermore, the groove 49 is tilted by 8° to make the half-wave plates 47, 48 tilt by 8° as shown in FIG. 4, to prevent loss due to reflections by the half-wave plates 47, 48. The center portion of the groove 49 (i.e. a portion facing to the Y-shape waveguide 3 where marked as section Z in FIG. 1), is filled with a resin 60 to block the leaked light from the Y-shape waveguide 3 more effectively, as shown in FIG. 6.

In the delay demodulation device 1, DQPSK signals transmitted from the optical fiber transmission line 54 to the optical receiver 50 are splits by the Y-shape waveguide 3, and the split DQPSK signals propagate to the two arm waveguides 8, 9 (wherein the lengths are different) of the first Mach-Zehnder interferometer. The Mach-Zehnder interferometer 4 shifts the phase of the DQPSK signal transmitted in one arm waveguide 8 by one symbol (i.e. π radians) with respect to the phase of the signal in the other arm waveguide 9. Similarly, the second Mach-Zehnder interferometer 5 shifts the phase of the DQPSK signal transmitted in one arm waveguide 12 by one symbol (i.e. π radians) with respect to the phase of the signal in the other arm waveguide 13.

The delay demodulation device 1 adjusts PDλ, for example, by using the heaters A, C or the heaters B, D of the Mach-Zehnder interferometer 4. After the adjustment, the delay demodulation device 1 performs phase shift control (or phase shift trimming) to shift the phases of one Mach-Zehnder interferometer by λ/2 radians to the phases of the other Mach-Zehnder interferometer, for example, by using the heaters A and C.

Embodiments

Delay demodulation device 1 has a PLC 1A on a silica base plate 30 shown in FIG. 3. The PLC 1A comprises: an input waveguide 2; a Y-shape waveguide 3; Mach-Zehnder interferometers 4, 5; output waveguides 21˜24 and two delayed portions 41, 43, wherein all of the components are made from silica glass. To create the demodulation device 1, FHD method, photo lithography, and reactive ion etching are used.

In the manufactured delay demodulation device 1, the difference in the refractive indexes between the cladding layer and the core layer (specific refractive index difference Δ) is 1.5%, and the size of the circuit (i.e. PLC chip 1B) is 25 mm by 25 mm.

To reduce the PDλ, heaters on one of the two Mach-Zehnder interferometers 4, 5 are used. Later, half-wave plates 47, 49 are inserted in groove 49, and phase shift control (or phase shift trimming) is performed by using heaters to shift the phase of one Mach-Zehnder interferometer by π/2 radians to the phase of the other Mach-Zehnder interferometer. To create a packaging, a fiber array comprising four optical fibers in a line is connected to one side 1 a of the PLC chip 1B. The side 1 a has the ends (i.e. output ports) of output waveguides 43˜46, which output optical signals to the outputs 1˜4, respectively. Also, as a temperature control device, a peltier element and a thermostat are used.

FIG. 7 is a result of temperature characteristics of the manufactured delay demodulation device 1. In FIG. 7, the starting value of the center wavelength is set to be the center wavelength of the through port (the output end 22 of the output coupler 7) on the Mach-Zehnder interferometer 4 when control temperature of the delay demodulation device 1 is set to 45° C. In FIG. 7, fluctuations from the starting value are shown. As shown in FIG. 7, within 0˜70° C. range, the fluctuations are within ±0.5 μm.

FIG. 8 is a graph showing a spectrum of a delay demodulation module includes the delay demodulation device 1. In FIG. 8, optical signals (a), (b) of the outputs 1, 2 are outputted from the output ends (i.e. the first and second output ports) of the waveguides 43, 44 of the first delayed portion 41, respectively. Similarly, optical signals (c), (d) of the outputs 3, 4 are outputted from the output ends (i.e. the third and fourth output ports) of the waveguides 46, 45 of the second delayed portion 42, respectively. As shown in FIG. 8, the optical signals (a), (b) are shifted by π radians with respect to the optical signals (c), (d). Furthermore, the optical signals (a), (b), (c), (d) are shifted by π/2 radians with respect to the adjacent optical signals. As shown in FIG. 8, desired characteristics of less than 3 pm (i.e. 0.003 nm) PDλ are obtained.

According to the embodiment presented above, the following advantages can be achieved.

Both ends of arm waveguides 8, 9 on the first Mach-Zehnder interferometer 4 and both ends of arm waveguide 12, 13 on the second Mach-Zehnder interferometer 5 are angled toward the center portion of the Planar Lightwave Circuit (PLC) 1A. Because of the construction, the lengths of two arm waveguides 8, 9 of the Mach-Zehnder interferometer 4 and the lengths of two arm waveguides 12, 13 of the Mach-Zehnder interferometer 5 in the Z-direction can be shortened. Also, because the directions of input couplers 6, 10 and output cuplers 7, 11 of Mach-Zehnder interferometers 4, 5 (i.e. direction of DQPSK signal transmitted in couplers) are tilted toward the Z-direction, lengths of the input couplers 6, 10 and the outputs couplers 7, 11 on Mach-Zehnder interferometers in the Z-direction can be shortened.

Because the lengths of the two arm waveguides on each Mach-Zehnder interferometers 4, 5 in the Z-direction and the lengths of the input couplers and the output couplers on Mach-Zehnder interferometers 4, 5 in the Z-direction are shortened, areas covered by the first and second Mach-Zehnder interferometers 4, 5 can be made smaller; and therefore, the PLC 1A of the delay demodulation device 1 can be made smaller. This also reduces temperature and stress distributions within the PLC. Therefore, the delay demodulation devices with little or no wavelength shift due to the environment and temperature fluctuation and small initial PDλ can be made.

The two waveguides 14, 15 split by the Y-shape waveguide 3 are bent upward and downward, respectively, to create a space between the two, and the third and fourth output waveguides 23, 24 are bent downward and upward, respectively, to come close to each other. According to the construction above, because the length of the two waveguides 14, 15 split by the Y-shape waveguide 3 and the length of the four output waveguides on the Z-direction are shortened, the PLC 1A of the delay demodulation device 1 can be made smaller to further reduce temperature and stress distributions within the PLC.

The first Mach-Zehnder interferometer 4 is at the upper-center portion of the PLC 1A, and the second Mach-Zehnder interferometer 5 is at the lower-center portion of the PLC 1A. Therefore, the PLC can be made even smaller.

A first delay portion 41 is placed in the first and second output waveguides 21, 22 to match the lengths of those two output waveguides 21, 22. Also, a second delay portion 42 is placed in the third and fourth output waveguides 23, 24 to match the lengths of those two output waveguides 23, 24. Because of the construction, the first delayed portion 41 matches the length of the first output waveguide 21 with the second output waveguide 22, and the second delayed portion 42 matches the length of the third output waveguide 23 with the fourth output waveguide 24. Therefore, the optical signals from the four output waveguides are shifted by π/2 radians with respect to the adjacent optical signals. In other words, output ends (four output ports) of the waveguides 43˜46 connected to the output waveguides 21˜24 output optical signals, which are shifted by π/2 radians with respect to the adjacent optical signals. Because of that, four-fiber fiber array can be directly connected to one side 1 a of the PLC chip 1B, which includes the outputs of the waveguides 43˜46.

The groove 49 for inserting the half-wave plates 47, 48 is placed on the straight line M-L in FIG. 1, which passes through the center portions of Mach-Zehnder interferometers 4, 5. Also, the groove 49 is placed on opposite side of the input waveguide 2 with respect to the Y-shape waveguide 3. Because of the construction, leaked light from the Y-shape waveguide 3 can be blocked. This prevents recombination of leaked light (DQPSK signal) from the Y-shape waveguide 3 with outputs of the output waveguides (output ends of the waveguides 43˜46).

The center portion of the groove 49 is filled with a resin 60 to block the leaked light from the Y-shape waveguide 3 more effectively.

The groove 49 is tilted by 8° to make the inserted half-wave plates 47, 48 to be tilted by 8° to prevent loss due to reflections by the half-wave plates 47, 48.

Because the half-wave plates 47, 48 are placed at the center portions of each Mach-Zehnder interferometer 4, 5, PDX can be reduced.

Because the center portion of the two arm waveguides 8, 9 of the first Mach-Zehnder interferometer 4 and the center portion of the two arm waveguides 12, 13 of the second Mach-Zehnder interferometer 5 are parallel to each other, size toward parpendicular to the Z-direction (outer volume) becomes smaller; therefore, the PLC 1A can be made even smaller.

The delay demodulation device 1 with an approximately square-shape PLC chip 1B can be obtained.

The delay demodulation device 1 shown in FIG. 1 includes two delayed portions 41, 42; however, the present invention can be applied to delay demodulation devices, which do not include any delayed portions. If the delay demodulation device does not have any delayed portions to match the lengths of output waveguides 21-24, then delayed portions are placed outside of the PLC chip 1B. Furthermore, the fiber array is connected to the two delayed portions placed outside of the PLC Chip 1B. The present invention is not limited to the above described embodiments and various and modifications may be possible without departing from the scope of the present invention. 

1. A Planar Lightwave Circuit (PLC) for demodulating Differential Quadrature Phase Shift Keying (DQPSK) signals comprising: an input waveguide, which receives the DQPSK signals; a Y-shape waveguide, which splits the input waveguide; a first Mach-Zehnder interferometer comprising: an input coupler, which is connected to one of the two waveguides split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths with respect to each other and connected between the input coupler and the output coupler, and a second Mach-Zehnder interferometer comprising: an input coupler, which is connected to the other waveguide split by the Y-shape waveguide; an output coupler, which is connected to output waveguides; and two arm waveguides having different lengths with respect to each other and connected between the input coupler and the output coupler; wherein both ends of the arm waveguides on the first Mach-Zehnder interferometer and both ends of the arm waveguides on the second Mach-Zehnder interferometer are bent toward a center portion of the PLC.
 2. The PLC of claim 1, wherein one of the two waveguides split by the Y-shape waveguide is bent upward and the other waveguide is bent downward to create a space between the two; wherein the waveguide bent upward is connected to an input end of the input coupler of the first Mach-Zehnder interferometer, and the waveguide bent downward is connected to an input end of the input coupler of the second Mach-Zehnder interferometer; and the first and second output waveguides connected to the output coupler of the first Mach-Zehnder interferometer are bent downward, and the third and fourth output waveguides connected to the output coupler of the second Mach-Zehnder interferometer are bent upward to come close to each other.
 3. The PLC of claim 1, wherein one of the two arm waveguides of each Mach-Zehnder interferometer has a difference in length Δ L with respect to the other arm waveguide of each Mach-Zehnder interferometer to create a phase shift of the DQPSK signal in one arm waveguide delay of π radians with respect to the other.
 4. The PLC of claim 1, wherein the input waveguide, the Y-shape waveguide, and the first and second Mach-Zehnder interferometers are waveguides placed on a base plate.
 5. The delay demodulation device of claim 4, wherein the base plate has an approximately square planar shape.
 6. The PLC of claim 1, wherein the first Mach-Zehnder interferometer is at the upper center portion of the PLC, and the second Mach-Zehnder interferometer 5 is at the lower center portion of the PLC.
 7. The PLC of claim 1, wherein the center portion of the arm waveguides of the first Mach-Zehnder interferometer and the center portion of the arm waveguides of the second Mach-Zehnder interferometer are extended in parallel to each other.
 8. The PLC of claim 1, wherein a first delay portion is placed in the first and second output waveguides to match the lengths of those two output waveguides, and a second delay portion is placed in the third and fourth output waveguides to match the lengths of those two output waveguides.
 9. The PLC of claim 1, wherein at least one heater is placed on at least one of the two arm waveguides of each Mach-Zehnder interferometer.
 10. The PLC of claim 9, wherein a half-wave plate is inserted at the center portion of each Mach-Zehnder interferometer.
 11. The PLC of claim 10, wherein a groove for inserting the half-wave plate, which passes through the center portions of the first and second Mach-Zehnder interferometers, is placed on the opposite side of the input waveguide with respect to the Y-shape waveguide.
 12. The PLC of claim 11, wherein a portion facing to the Y-shape waveguide is filled with a resin. 